1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus and an image correcting method capable of compensating for the deterioration of horizontal charge transfer efficiency conspicuous in an image output in, e.g. a high ISO (International Standards Organization) sensitivity mode.
2. Description of the Background Art
A solid-state image pickup apparatus of the type operable in a high ISO sensitivity mode is conventional. With this type of image pickup apparatus one of the problems is that the level of signals output from an image sensor after picking up an image decreases with the image sensor having an increased number of pixels and a decreased size of the individual pixel. Another problem is that during the horizontal transfer of signal charges in the image sensor the amounts of signal charges received from the preceding pixel and of signal charges left in the following pixel have the characteristic curve thereof upwardly convex with respect to the signal level, so that the ratio of a transfer remainder to the original signal in the form of an offset increases to critically degrade colors. This is particularly conspicuous when the amount of signal charges to be transferred is smaller.
Usually, the image sensor is made up of an image sensing surface or photosensitive array, a horizontal transfer path, a floating diffusion amplifier (FDA) and an output circuit. Signal charges of respective pixels are vertically transferred to the horizontal transfer path, which horizontally transfers the signal charges of each row to a concentrating portion. Similarly to the horizontal transfer path, the concentrating portion sequentially transfers the signal charges of each row in the horizontal direction.
However, the horizontal transfer path and the concentrating portion may have lattice defects caused in the course of the manufacture process for the image sensors. The number and the location of the lattice defects are varied from one image sensor to another. Due to these lattice defects, the horizontal transfer path and the concentrating portion sometimes leave over signal charges in the course of the horizontal transfer, thus deteriorating the horizontal transfer efficiency.
When an image sensing surface which has its pixels or photosensitive cells arrayed in a so-called “honeycomb” pattern generates signal charges representing the gray incident light and transfers them to the horizontal transfer path, specifically, the signal charges of a row of red (R) and blue (B) pixels on the horizontal transfer path, the R pixel data 302 and the B pixel data 304 are approximately equal to each other in signal level, as seen from FIG. 6. Thus, in this case, a scrutinization of the relationship of correspondence between the signal quantity of a given pixel and the remainder signal quantity to the next pixel, that is, the horizontal transfer efficiency 310, may reveal that the remainder quantity from the R pixel to the following pixel is approximately equal to that from the B pixel to the following pixel, as understood from FIG. 7.
By contrast, when signal charges of a row of R and B pixels, representing the red incident light, are generated and transferred on the horizontal transfer path, the R pixel data 322 are appreciably higher than the B pixel data 324 in signal level, as understood from FIG. 8. Thus, if the horizontal transfer efficiency 330 is scrutinized in this case, the remainder quantity 332 from the R pixel to the next pixel is larger than that from the B pixel data to the next pixel, as seen from FIG. 9.
Thus, the adverse effect of deterioration in the horizontal transfer efficiency, that is, the remainder quantity with respect to the signal quantity is generally higher for a subject with chromatic color to be shot, depending on how the pixels are arrayed in the horizontal direction. It may be surmised that signal charges of a row of green (G) pixels are of the same level and hence are not affected by deterioration in the horizontal transfer efficiency.
Meanwhile, in an electronic camera, disclosed in U.S. patent application publication No. 2005/0036040 A1, the R/G ratio and the B/G ratio are acquired as reference image sensing conditions, from the output signal of the image sensor, and the status of mixed colors is verified responsive to the reference image sensing conditions to decide the gain in the gain control. The gain may be determined from one color signal to another.
In light of the above, Japanese patent laid-open publication No. 2004-327722 discloses a solid-state image sensor manufactured in such a way that, in the manufacturing process, image data A and B are output from two kinds of photodiodes having the storage capacities of signal charge thereof different from each other, mean values A1 and B1 of differences between signals output from two nearby pixels around a concentrating portion are calculated, mean values A2 and B2 of differences of signals output from two nearby pixels at a portion remote from the concentrating portion are calculated, a ratio T=A1/B1 is used to determine whether or not the defect of the concentrating portion is acceptable, and then the ratio T and a ratio U=A2/B2 are used to determine whether or not the defect of the horizontal transfer path is acceptable.
Meanwhile, when the signal quantity generated in response to a light volume incident on the image sensing surface is scrutinized, it is seen that the more the incident light volume, that is, the lower the value of ISO sensitivity, the more becomes the signal quantity, as shown in FIG. 10, whereas the lesser the incident light volume, that is, the higher the value of ISO sensitivity, the lesser becomes the signal quantity.
More specifically, if the relationship of the remainder quantity with respect to the signal quantity, that is, the horizontal transfer efficiency, is scrutinized, as seen from FIG. 11, then it may be seen that, since the remainder quantity has its characteristic curve 340 upwardly convex, the remainder quantity ΔSa is decreased for a larger signal quantity, that is, for the lower-value of ISO sensitivity, while the remainder quantity ΔSb is increased for a smaller signal quantity, that is, for the higher value of ISO sensitivity. Thus, the horizontal transfer efficiency is lowered significantly. Hence, in compensating the effect of deterioration in the horizontal transfer efficiency, correction must be effected more strongly when the value of ISO sensitivity is high than when the value of ISO sensitivity is low.
Moreover, in the electronic camera, described in U.S. patent application publication No. 2005/0036040 A1, the amplification factor of the gain controller is corrected or modified depending on the sensitivity derived from an output signal of the image sensor, that is, on the ISO sensitivity. However, this does not provide for impeccable color correction of the generated image. On the other hand, the deterioration of the horizontal transfer efficiency worsens the color cast of an image captured with a high value of ISO sensitivity, and hence the deviation of color patch for an image becomes particularly larger, with the result that the correction of a single color, such as gray, is not sufficient.